It's tough. Logbook? Keep track of the type, duration, and intensity of exercise in a log book. Use this to track your progress/improvement. Depending on your type of exercise, make one or more periodic adjustments weekly: may increase speed, increase duration, increase incline, amount of weight, number of weight sets, swim laps, etc.
A log book is more helpful than it sounds.
Also, use a buddy- motivates both.